I'm planning on using the dremmel for grinding out some blisters and voids in the fiberglass matt of the boat bottom. The only concern is that due to the high rev, the fiberglass will tend to be melted and that will inhibit any penetration of the epoxy repair into the mat. Not sure if that is significant. All the gelkote has been peeled and there only remains some remaining blisters in the chopped mat.

I just finished closing 2 thru-hulls for an old toilet I am no longer using. I used epoxy directly on top of sanded fiberglass after I filled the holes from the inside 1st. I sanded the area to scuff it up for adhesion, wiped with acetone and applied my resin. After it cured, the next day, I sanded it smooth and the bond was excellent. I feathered the edges so thin you can't feel them at all with your fingers. What you are doing here is applying a thin layer or "Film" of epoxy over the area you wish to repair. If the surface underneath is clean fiberglass, the epoxy WILL bond. I use the MAS brand "Flag" epoxy and swear by it. I use SLOW hardener to give me plenty of time to work, but if its cooler where you live go with a faster hardener. My boat was built in 1967 and the glass is pretty stout. I trust my repairs since my holes were quite large and well below the waterline. (btw- I did reinforce with layers of cloth on both sides, since I had so much area to cover)

I used a round tipped burr on a drill to do mine. It chewed through the glass pretty easily and left a nice rough surface for bonding. I have tried rotary wood rasps on glass too, but the ones I had were cheap and wore out quickly.

I doubt you are going to melt the fiberglass with high speed, but you don't need 10,000 rpm! Just slow it down a bit!

Trouble is that I have a single speed dremel and I think its' rpm is around 35k so melting is surely possible. Maybe roughing it up with 80 grit after grinding?..or just getting an attachment for my varible speed drill

Trouble is that I have a single speed dremel and I think its' rpm is around 35k so melting is surely possible. Maybe roughing it up with 80 grit after grinding?..or just getting an attachment for my varible speed drill

Yes, in that case the Dremel is not the right tool for the job. Just use a regular drill. You don't need a special attachment, just buy a burr bit and it will fit in any drill chuck.

Perhaps a bit overkill for small blisters but they make a tool called a 4-1/2" angle grinder, or grinder that with a #60 grit circular sanding disk on it will take down the laminate in no time. You'd have to use a very light touch with it though.

I like the idea of leaving a rough surface for the new epoxy to bond to.
SchockT, do you have a link to this 'burr bit' you mentioned?

The burr I used was one that has been in my tool box for years. They are very common if you go to any good tool store and come in a huge variety of different shapes. I used one that was cylindrical with a round head. It worked really well because you could grind in a very localized area. On the smaller blisters is was just a matter of pressing the tip into the blister and it's gone!

The fourth from the left on the second row is similar to the one I used.

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